1. Introduction
With the continuous improvement in people’s day—to—day life and industry production levels, the pollution of water and sediment by heavy metals is also deepening. At the same time, the sources of heavy metals are wide—ranging, mainly from industrial production industries such as the machinery processing industry, steel, non—ferrous metal smelting industry, etc. In addition, the emission of automobile exhaust, the disposal of waste batteries, and the abuse of agricultural fertilizers in our daily life can also cause serious heavy metal pollution to river—bottom mud [1,2].
The composition of the surface river and lake sediments is complex, mainly including clay, sediment, organic matter, and various mineral bodies [3,4]. Heavy metals in sediment mainly originate from polluted water bodies, and a series of transformation and migration processes such as adsorption–desorption, precipitation–dissolution, complexation–decomplexation, ion exchange, and redox occur between the sediment and the water body [5].
Because of their slow mobility and long residence time, heavy metals in the sediment are not degraded easily by microorganisms [6], and if they are not treated in time, they will become a “new source of pollution”, polluting the water body twice, poisoning the plants and animals in the water and then further endangering human life and health through transmission via the food chain [7]. The problem of how to treat heavy metals in river sediments and water bodies has become a pressing one [8].
Long—term exposure to heavy metal ions can cause serious harm to the human body, e.g., the activity of enzymes, the nervous system, and some organs with detoxification functions will be affected [9]. The use of water contaminated with heavy metals can cause direct harm to the human body; for example, in the 1950s and 1960s, Japanese production mines exceeded the Cd(II) standard in nearby waters, causing some nearby residents to suffer from “bone pain”, leading to osteochondrosis and general pain. Further, the accidental release of chemicals containing Hg(II) in Taiwan has contaminated the soil in several locations, and the consumption of crops grown on these soils can cause the heavy metal Hg(II) in the human body to exceed the limit. When people consume Hg(II)—poisoned fish, they will suffer from neurological toxicity, impaired sensation at the ends of their limbs, and narrowed vision [10].
Adsorption is the most commonly used method for treating heavy metals in water bodies and sediments, with the main advantages of being renewable, low cost, creating no “secondary pollution”, and suitable for treating various concentrations of wastewater.
Biochar is a carbon—rich product obtained by charring biomass, and it has been widely studied for its advantages of low price and a wide range of sources. With its large porosity as well as specific surface area and the presence of many functional groups on its surface, it is an efficient and low—cost adsorbent and is often widely used to remove heavy metal pollutants from water [11]. Singh et al. conducted batch experiments on Cu(II), Cr(VI), Cd(II), and Pb(II) to investigate the effects of contact time, adsorbent dose, pH, and charring temperature on the adsorption of heavy metals. The experimental results showed that the adsorption capacity was positively correlated with the adsorption time in the first 20 min but gradually tended to reach equilibrium after 20 min. The adsorbent dose and pyrolysis temperature were positively correlated with the removal effect of biochar for heavy metals, and the maximum removal efficiency of biochar could reach 99.86% at pH = 4 [12]. Park et al. investigated the adsorption of heavy metals on sesame straw charcoal (SSB) and compared its adsorption of single and multiple metals. They found that SSB had superior adsorption effects on heavy metals, and the adsorption capacities of SSB for single heavy metals were, in descending order, Pb(II) > Cu(II) > Cr(VI) > Zn(II) > Cd(II), and for polymetals were, in descending order, Pb(II) > Cd(II) > Cr(VI) > Cu(II). The adsorption capacity for polymetallics was much greater than for monometallics [13]. Although biochar has a certain adsorption effect on heavy metals, the adsorption capacity is relatively limited, and in order to improve the utilization of biochar, biochar should be further modified to effectively enhance its remediation function [14].
2. Modified Carbon Properties and Types
Biochar is a black carbon—containing solid obtained by high—temperature pyrolysis of biomass under anaerobic or oxygen—limited conditions. Biochar contains a large number of pores and rich functional groups, with a large specific surface area and high stability, and has been widely used in soil remediation, carbon sequestration, and wastewater treatment [15]. However, the light mass, small powder particles, and single composition of biochar compromise the effectiveness of the adsorption of heavy metals and organic pollutants, etc., and recovery in water and soil is limited. Modified biochar can compensate for some of the shortcomings of single biochar and improve the adsorption effect, so modified biochar materials have received much attention in recent years.
There are usually two types of modifying biochar: one is to modify the prepared biochar by impregnation with chemicals or co—precipitation with metals; the other is to prepare biomass raw materials by mixing them with modification reagents and preparing them by high—temperature pyrolysis. Biochar modification includes surface structure modification and surface chemical modification. Surface structure modification is mainly to change the pore structure of biochar to increase the specific surface area in order to increase the adsorption capacity; surface chemical modification is to modify the functional groups on the surface of biochar to increase the adsorption sites and improve the adsorption effect. The general preparation process of mixing biomass raw materials with other reagents for pyrolysis modification includes mixing, filtration, drying, pyrolysis, and debinding.
3. Mechanism of Heavy Metal Adsorption by Biochar and Modified Biochar
The main adsorption mechanisms of biochar and modified biochar for heavy metals include physical adsorption and chemical adsorption. Physical adsorption is mainly based on the specific surface area or van der Waals force. However, they can be ignored because of their weak effects [16]. The specific adsorption mechanisms of biochar and modified biochar for heavy metals are shown below.
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(1). Surface adsorption
The surface of biochar contains many acidic groups, such as carboxyl, phenolic hydroxyl, etc., which can form specific metal complexes with heavy metal ions in water and soil to form active adsorption sites, so that surface adsorption can be carried out. The main influencing factors are the chemical bond on the surface of biochar, the diffusion effect of heavy metal ions, etc. [17,18,19,20]. Linbo Qian et al. used rice straw to prepare biochar colloids and found that the main mechanism of adsorption was that the surface of the biochar gel made from rice straw had a large number of oxygen—containing functional groups and minerals at a high carbonization temperature, which could be used as adsorption sites for Cr(III) and Cd(II) [20].
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(2). Electrostatic adsorption
Electrostatic adsorption is the essence of forming ionic bonds, which can be influenced by zeta potential and pH [21]. Chen Zhuang et al. used iron nitrate—modified reed biochar (FeBC) and found that the removal rate of the modified charcoal for Cr(VI) could reach 94.52%, where the adsorption mechanism was mainly due to the electrostatic adsorption of positive charges on the surface of modified FeBC with Cr(VI) and Cd(II) [22]. Xuejiao Tong studied the effect of zeta potential and pH on the electrostatic adsorption of crop straw char on heavy metal Cu(II), and the experimental results showed that the absolute value of the negative zeta potential increased with the increase in pH, i.e., more and more anions on the biochar surface, thus helping with the adsorption of Cu(II) by the biochar [23].
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(3). Ion exchange
The essence of ion exchange is the exchange reaction between charged cations and protons on the surface of biochar and dissolved heavy metal ions under suitable pH conditions [24,25]. The main biochar surface cations that can exchange ions with heavy metal ions are potassium, calcium, sodium, and magnesium ions. The nature of the functional groups on the surface of biochar, the size of the contaminants, and the nature of the charge all have an effect on ion exchange. Li A Y et al. prepared six types of biochar (BSB, CSB, FSB, CFSB, MSB, and TSB) with different raw materials, in which magnesium ions were loaded by the impregnation method. The experimental results showed that these types of biochar had good adsorption effects on heavy metals such as Cd(II), Cu(II), and Pb(II). Biochar contains a large number of magnesium ions and functional groups that can form complexes with metal ions to perform a strong ion exchange. Therefore, mineral precipitation and cation exchange play a dominant role in the adsorption process [26]. However, when the concentration of metal ions such as potassium, calcium, sodium, and magnesium ions on the surface of biochar is too high, it will compete with heavy metal ions, thus reducing the effect of ion exchange and hindering the adsorption process of heavy metals, so the effect of ion exchange is related to the concentration of metal ions on the surface of biochar [27,28].
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(4). Chemical precipitation
The minerals in biochar contain anions such as OH−, SO42−, SO32−, and CO32−, which provide a large number of adsorption sites for heavy metals and react with them to form water—insoluble precipitates [29,30]. When the elemental composition of biochar has a high content of P, S, Si, and Al, it might mean that there are more mineral acid ions on its surface, which can have a positive effect on the removal of heavy metals by biochar. Xiaoyun et al. prepared rice husk biochar (RHBC) and cow dung biochar (DMBC) using rice husk and cow dung as raw materials and used them as adsorbents to remove Pb(II), Cu(II), Zn(II), and Cd(II) from aqueous solutions. The experimental results showed that DMBC had better adsorption capacity for these four heavy metals compared to RHBC, and its adsorption mechanism was mainly due to the fact that the surface of DMBC was rich in anions such as CO32− and PO43−, which participated in the adsorption reaction and provided reaction sites in contact with Pb(II) and further formed precipitation, thus removing Pb(II) from water [31]. Zama, E.F et al. investigated the effect of biochar prepared from various raw materials at different charring temperatures for the removal of heavy metals Pb(II) and Cd(II); the experimental results showed that the best adsorption effect for Pb(II) and Cd(II) was achieved when the charring temperature was 700 °C for rice stalk charcoal and the adsorption capacity could be distributed up to 126.58 mg/g and 60.61 mg/g, as shown by the XRD pattern. The analysis showed that the biochar indicated the presence of a large amount of CO32−, PO43−, and SiO44, which can produce chemical precipitation with heavy metals [32].
The specific mechanisms of biochar with heavy metals are shown in Figure 1, the behavior of biochar with reference to metals are shown in Figure 2 and Figure 3, and the adsorption mechanisms and principles of biochar, as well as modified biochar, are shown in Table 1:
The surface of biochar is smooth in the macro view, but in fact, its surface is often uneven at the atomic level. The atoms and molecules on the solid surface are subject to uneven forces, but because the position of the molecules on the solid surface is fixed and cannot be moved like the molecules on the liquid surface, the uneven forces are difficult to balance.
Due to the asymmetry of the force exerted on the atoms on the solid surface and the non—uniformity of the surface structure, the solid surface layer material receives a pull pointing inward, resulting in an unbalanced force field.
Because the atoms on the solid surface are positioned, it is not possible to balance the force field by shrinking the surface, but it is possible to use the residual force on the surface to capture other metal ions from the surrounding medium so that the force imbalance can be compensated to a certain extent, and adsorption can be generated.
4. Biochar Modification Methods and Their Principles
Although biochar has a certain adsorption effect on heavy metals, the adsorption capacity is relatively limited. In order to improve the utilization of biochar, it is necessary to modify biochar further to effectively enhance the remediation function of biochar [33]. The modification methods of biochar include physical modification, redox modification, acid–base surface modification, adsorbent compound modification, and activation modification. These methods can improve the adsorption capacity of biochar for heavy metals by increasing the number of functional groups on the surface of biochar, the biochar’s specific surface area, and pore capacity. The specific modified materials and modification principles used in each modification method are shown in Table 2.
5. Application of Biochar and Modified Biochar to Remove Heavy Metals from Water
(1) Removal of heavy metals from water by biochar
Biochar is a carbon—rich product obtained by charring biomass that has high porosity and a large specific surface area, and many functional groups exist on its surface, making it an efficient and low—cost adsorbent that is often widely used to remove heavy metal pollutants from water. The raw material selected for biochar and the charring temperature are the key factors that determine the chemical and physical properties of the biochar [41].
The effectiveness of biochar in adsorbing heavy metals in water varies depending on the raw material. A study showed that the adsorption of heavy metal Cu(II) by coconut shell charcoal, bamboo charcoal, and wood charcoal was effective; the adsorption processes of coconut shell charcoal and bamboo charcoal were more consistent with the results of the fitted Freundlich isothermal adsorption model, and the adsorption of wood charcoal was more consistent with the results of the fitted Langmuir isothermal model [42]. Shaon Kumar Das et al. selected four kinds of raw materials, such as corn stalks as raw materials and prepared four different types of biochar for the adsorption of Cd(II), Pb(II), Ni(II), Zn(II), and Cd(II) from water at a charring temperature of 600 °C. The experiments showed that each biochar was effective in the removal of heavy metal mixtures from water. The adsorption capacity of the biochar made from different materials varied for each metal. The removal rates of heavy metals by corn stalk charcoal, pine needle tree charcoal, BGB, and LCB were Cu(II) > Pb(II) > Zn(II) > Cd(II) > Ni(II); Ni(II) > Cd(II) > Zn(II) > Cn(II) > Cu(II); Cd(II) > Zn(II) > Pb(II) > Cu (II) > Ni(II); and Cu(I) > Cd(I) > Pb(I) > Zn(I), respectively [43].
Charring temperature is also a key factor affecting the ability of biochar to adsorb heavy metals. Jia Y et al. used rice husk and cotton straw to make biochar at charring conditions of 300, 400, 500, 600, and 700 °C. The experimental results showed that the charring temperature was negatively correlated with the biochar yield and positively correlated with the ash content, and the pH also changed from acidic through neutral to alkaline or even strongly alkaline with the increase in charring temperature, while the aromaticity of the biochar gradually increased, the number of oxygen—containing functional groups decreased and the structure became more stable, but the hydrophilicity and polarity decreased [44]. Xiao Y et al. used 300, 450, and 600 °C for charring to determine the optimal temperature to improve the binding capacity of biochar. The experimental results showed that the charring temperature had different effects on the elemental composition, surface area, and active functional groups of the resulting biochar and that the biochar produced at a charring temperature of 600 °C had the strongest adsorption capacity for Pb(II), which could reach 190.7 mg/g [45].
(2) Removal of heavy metals from water by modified biochar
Zhao Jie et al. used HNO3, H3PO4, NH3—H2O, and Ca(OH)2 to modify biochar at a carbonization temperature of 400 °C. The results of acid modification showed that the acidic functional group content in the HNO3—modified biochar and H3PO4—modified biochar increased and the pH of the acid—modified biochar decreased, and its corresponding pHpzc increased, while the results of the alkaline—modified biochar and the acid—modified biochar were diametrically opposed. The adsorption of Cr(VI) by acid—modified biochar was better than that by alkaline—modified biochar, and the best adsorption capacity of Cr(VI) by H3PO4 modified biochar was 101.82 mg/g, while that of unmodified biochar was 58.48 mg/g, indicating that the adsorption capacity of modified biochar for heavy metals was enhanced [46]. The Douglas fir biochar (DFBC) obtained by rapid charring at high temperatures was modified by KOH activation by Herath et al. The experimental results showed that the surface area of KOH—activated modified biochar (KOHBC) increased from 535 m2/g without modification to 1049 m2/g, and the adsorption capacities for heavy metals Pb(II), Cr(II), and Cd(II) were up to 140.0, 127.2 and 29.0 mg/g, respectively, at the optimum pH conditions for each heavy metal [47]. Qu et al. investigated the effect of KOH activation on the removal of Cr(VI) from water using corn straw as the raw material and biochar produced by a two—step carbonization process. The experimental results showed that KOH—activated modified carbon has a large specific surface area and many developed micropores, and its theoretical monolayer adsorption capacity for Cr(VI) can reach 116.97 mg/g, with the adsorption mechanisms mainly being electrostatic attraction, complexation, ion exchange and reduction [48].
6. Application of Biochar and Modified Biochar for Curing Heavy Metals in Soil
(1) Biochar solidification of heavy metals in soils
Biochar can fix heavy metals in contaminated substrates and soils because the products made from different biomass materials at suitable temperatures have a rich pore structure and are rich in surface functional groups such as carboxyl groups, hydroxyl groups, and amides. It can not only improve the fertility and permeability of the soil but also forms a coordination effect with a large number of heavy metal ions, effectively reducing the mobility of heavy metals in the substrate and soil and slowing down the toxic effects on plants [49].
Biochar is generally alkaline and can effectively raise the pH of the substrate and soil, which has a positive effect on the solidification of heavy metals contained in them. Its effectiveness in fixing heavy metals in contaminated substrates and soils is mainly influenced by the charring temperature and biomass [50]. A higher charring temperature usually increases the surface area and charring rate of biochar, which increases the adsorption capacity of pollutants and can reduce the bioavailability of heavy metals.
The specific surface area, porosity, and the number of functional groups of biochar from different sources also differed, which helped the biochar to solidify the heavy metals within it [51,52]. The experimental results showed that rice husk biochar could reduce the acid—soluble fraction of the contaminated sediment by 18–31%, while moso bamboo charcoal could reduce the acid—soluble fraction of Cu(II) by 79.71% [53,54].
Biochar not only has a fixation effect on heavy metals in contaminated substrates and soils but also reduces the heavy metal content in plants growing in contaminated soils. Gong et al. used tea—waste—derived biochar (TB) for the phytoremediation of Cd(II)—contaminated sediments. The results showed that TB at application rates of 100, 500, and 1000 mg/kg increased the accumulation and translocation of Cd(II) in ramie seedlings by altering the morphology of Cd(II) in the sediment and changing the subcellular distribution of Cd(II) in the plant cells. Low levels of TB reduced the toxicity of Cd(II) to ramie seedlings by promoting plant growth and alleviating oxidative stress. The results showed that a low concentration of biochar could improve phytoremediation efficiency and reduce the toxicity of Cd(II) to plants and microorganisms in the soil [55].
(2) Modified biochar to solidify heavy metals in soil
Zou Qi et al. used red mud—modified biochar to immobilize Cd(II) from the subsoil of the Xiangjiang River, and their results showed that red mud—modified biochar significantly reduced the concentration of As in soil (p < 0.05) [56]. The experimental results showed that the overlying water and pore water concentrations of Cd(II) decreased by 71% and 49%, respectively, after the activated biochar treatment, and the bioavailability of Cd(II) decreased, which proved that the specific surface area and oxygen—containing functional group content of activated biochar were related to the immobilization of Cd(II) in sediments [57].
The modification of biochar with different modifiers showed different inhibitory effects on the release of heavy metals from the bottom sediment. Cao Jing et al. prepared four different types of modified biochar with FeCl2, AICI3, MgCl2, and KMnO4 as modifiers and investigated their effects on controlling the release of heavy metal ions into water from two types of polluted substrates in urban river networks and lakes in Jiaxing City. The study showed that the remediation effect of adding AICI3—modified biochar was the best for Ni(II) > As contaminated sediment, and the concentrations of Ni(II) and As released into water from the two contaminated sediments could be reduced at most by 72.13% and 46.21%, respectively; the concentration of As released into water from the two contaminated sediments could be reduced at most by 95.80% after adding FeCb—modified biochar, but the concentration of Ni(II) in water increased. After adding FeCl2—modified biochar, the concentrations of Ni(II) and As pollutants released from the two contaminated substrates into the water increased [58].
Modified biochar not only has a remediation effect on the soil contaminated with heavy metals but also reduces the heavy metal content in plants growing in the contaminated soil. Sun Tong et al. studied the passivation and remediation of weakly alkaline Cd (II) contaminated soil by calcium—based modified biochar and its effects on soil physicochemical properties and Cd (II) accumulation in plants. The effective Cd(II) content of the soil was reduced by 12–30.2%, and the morphology of Cd(II) in the soil changed from the more active exchangeable and reducible states to the more stable residue state. The Cd content in the kernels of Zhengdan 958, Liyu 16, and Sanbei 218 was reduced by 52.65–72.56%, 37.54–50.80%, and 23.6–51.2%, respectively, compared with the control before the application of the modified activated carbon [59].
7. Conclusions
Adsorption is nowadays the most commonly used method for treating heavy metals in water, which has the advantages of being renewable, having low cost, having no “secondary pollution” and being suitable for treating wastewater of various concentrations. Biochar adsorbent has become a common material for treating heavy metals in wastewater because it is environmentally friendly, low—cost, and easy to obtain. Although biochar has a certain adsorption effect on heavy metals, the adsorption capacity is relatively limited. In order to improve the utilization of biochar, it is necessary to modify biochar to effectively enhance its remediation function further. In this paper, we have analyzed different modification methods of biochar from the adsorption mechanisms of biochar, compared the application of biochar before and after modification for heavy metal adsorption in both water and soil, and detailed the treatment effects and influencing factors of biochar before and after modification.
In the future, we can explore new biomass carbonization materials and rationalize the use of renewable waste resources to find more suitable adsorbents. Meanwhile, modified biochar materials will be further applied to the treatment of heavy metals in water and soil in a reasonable way. It is also worth noting that, in the future, the preparation of selective adsorbent materials can be considered through optimal modification, which can be more efficiently applied to the treatment of water and soil pollution in mixed systems in actual production and life. In addition, in the context of a green and environmentally friendly environment, we should pay more attention to the repeated recycling of waste biomass to ensure a good treatment effect without causing secondary pollution to the environment.
Conceptualization, S.L. and H.L.; methodology, H.L. and Y.W.; validation, Y.W., H.L. and S.L.; formal analysis, Y.W.; resources, Y.W.; ing—review and editing, Y.W. All authors have reawriting—original draft preparation, Y.W.; writd and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
Not applicable.
The authors declare no conflict of interest.
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Adsorption mechanism of heavy metals by biochar and modified biochar.
| Mechanism | Principle | Main Influencing Factors | References |
|---|---|---|---|
| Surface |
The surface of biochar is rich in acidic groups such as carboxyl groups and phenolic hydroxyl groups, which can form specific metal complexes with heavy metal ions in water/soil and form active adsorption sites, etc. | 1. Surface chemical bond group |
[ |
| Electrostatic |
Formation of ionic bonds (formed when atoms gain or lose electrons) between anions and cations by electrostatic interaction (chemical bonding). | 1. Zeta potential |
[ |
| Ion Exchange | The charged cations and protons on the surface of biochar exchange with dissolved heavy metal ions in an exchange reaction. | 1.Nature of the surface functional groups |
[ |
| Chemical |
Anions react with heavy metal ions to form a water—insoluble precipitate. | 1. pH value. |
[ |
Modification methods and principles of modified biochar.
| Modification Method | Modified Materials | Modification Principle | References |
|---|---|---|---|
| Physical modification | CO2, H2O, Air | By reacting the gas with the biochar at high temperatures, the pores of the biochar are increased, and its specific surface area is expanded. | [ |
| Redox modification | FeCl3, MnSO4, CO2, NH3 | Using oxidizing and reducing agents to carry out redox reactions on the functional groups on the surface of biochar to increase the surface active matching point. | [ |
| Acid–base |
CO2, NH3, KOH, NaOH | Using acid and alkali treatment of raw materials or direct treatment of biochar to change the acid and alkali functional groups on the surface of the biochar. | [ |
| Adsorbent |
Nanocomposites such as chitosan | Nanocomposites such as chitosan are rich in amino—functional groups with strong binding capacity and thus can be used as adsorption sites for heavy metals. | [ |
| Activation modification | KOH, NaOH, ZnCl2, H3PO4 | The activator reacts with the functional groups on the surface of the biochar at high temperatures and introduces a large number of active sites, increasing the number of pores on the surface of the biochar and increasing the diameter of the pores. | [ |
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; He, Li 1 ; Lin, Shaohua 2 1 School of Civil Engineering, Southeast University, Nanjing 210096, China
2 School of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China